1. Field of the Invention
This invention relates to user equipment for communication systems and in particular, but not exclusively for high speed downlink packet access (HSDPA) for WCDMA communication systems.
2. Description of the Related Art
As is known in the field a further development of the wideband code division multiple access (WCDMA)/universal mobile telecommunications system (UMTS) communication system defined by the 3GPP organization, is the definition of the system known as high speed downlink packet access (HSDPA). HSDPA operates as a time shared communications channel which provides the potential for high peak data rates as well as the possibility for having a high spectral efficiency.
Current 3GPP HSDPA standards (e.g. 3GPP TS 25.858) define a HS-DSCH channel (high speed downlink shared channel), which is a downlink transport channel shared by several user equipment units. The HS-DSCH is associated with one downlink DPCH (downlink dedicated physical channel) or F-DPCH (option in 3GPP Rel6) per active user, and one or several shared control channels (HS-SCCH). The HS-DSCH can be transmitted over the entire cell or over only part of the cell using for example beam-forming antennas.
HSDPA improves system capacity and increases user data rates in the downlink, in other words for transmission of data from a radio base station (RBS) which in a UMTS system is also known as a node B server (and in the GSM by the term base transceiver station BTS) to the user equipment.
This improved performance is based on three aspects. The first aspect is the use of adaptive modulation and coding.
In HSDPA, the link adaptation entity in the radio base station (Node-B server) tries to adapt to the current channel conditions of a certain user equipment (or user terminal) by selecting the highest possible modulation and coding scheme keeping the frame error probability below a certain threshold. For that purpose, the user equipment periodically sends channel quality feedback reports to the respective serving RBS, which indicate the recommended transmission format for the next transmission time interval (TTI), including the recommended transport block size, the recommended number of codes and the supported modulation scheme as well as a possible power offset. The reported channel quality indicator (CQI) value is determined on the basis of measurements of a common pilot channel. In a typical implementation it is a pointer to an index in one of the tables specified in the document “3GPP TS 25.214—Physical Layer Procedures (FDD)” that define the possible transmission format combinations (as mentioned above) for different categories of user equipment (UE).
The second aspect is the provision of fast retransmissions with soft combining and incremental redundancy, so that should link errors occur the user equipment rapidly requests retransmission of the data packets. Whereas the standard WCDMA network specifies that the requests are processed by the radio network controller (RNC), in HSDPA the request is processed by the RBS. Furthermore the use of incremental redundancy, allows the selection of correctly transmitted bits from the original transmission and retransmission in order to minimize the need for further repeat requests when multiple errors occur in transmitted signals.
The third aspect of HSDPA is fast scheduling in the RBS. This is where data to be transmitted to the user equipment is buffered within the RBS prior to transmission and the RBS using a selection criteria selects some of the packets to be transmitted based on information about the channel quality, user equipment capability, the quality of service class and power/code availability. A commonly used scheduler is the so-called proportional fair (P-FR) scheduler.
Although HSDPA is an efficient method for delivering relatively large amounts of data in relatively small time periods (the TTI for a HSDPA system is 2 ms).
This performance however can only be used when the user equipment is operating within the dedicated channel state (CELL_DCH state), in other words after a physical layer connection between UE and the RBS has been established and the layer connection has dedicated channels allocated to it.
The transition of the UE to the dedicated channel state (CELL_DCH state) and establishing a HSDPA connection may take up to a second, Thus specifically where the amount of data required to be transmitted is relatively small the state transition to the CELL_DCH state can take longer that the actual data transmission.
Moreover, when the UE is in the process of changing state to the CELL_DCH state, the required state change has to be addressed to the UE by the forward access channel (FACH) which is significantly slower and less robust than the later HSDPA transmission channels.
Before and during the transition to the CELL_DCH state, the CELL_FACH state requires that both the downlink dedicated control channel (DCCH) and the downlink dedicated traffic channel (DTCH) are mapped onto the forward access channel (FACH). This requirement increases the radio resource control (RRC) signalling (caused by the extra DCCH information) and data (caused by the extra DTCH information) transmission delay. The minimum time duration of the FACH transmission (which is carried over the secondary common control physical channel (S-CCPCH)) is approximately 10 milliseconds.
During the radio resource control (RRC) connection establishment phase the common control channel (CCCH) transmission is mapped onto the forward access channel (FACH). FIG. 1 shows the procedure for transition of the UE to the dedicated channel state (CELL_DCH) as described in 3GPP technical report TR 25.931. In step 107 of FIG. 1, the RRC connection setup message which is typically carried over the common control channel (CCCH) is carried on the forward access channel (FACH) which in turn is mapped on the secondary common control physical channel (S-CCPCH).
It is also known to deliver data to a UE not in the dedicated channel (CELL_DCH) state by using the forward access channel (FACH) to deliver small amounts of data or control information to the UE. However this approach suffers from the inherent problems associated with the FACH, a low data rate and slow retransmission.
The capacity on the forward access channel (FACH) carried on the S-CCPCH is relatively low, typically between 32 to 64 kbps, which limits the use of the forward access channel to small packets.
Typically it is therefore only possible to transmit one or two common control channel (CCCH) radio link control protocol data units (RLC PDU) in a single TTI (a typical CCCH RLC PDU packet is 152 bits). Signalling radio bearers (SRB) mapped onto the dedicated control channel (DCCH) and utilising unacknowledged mode radio link control (UM RLC) packets produce RLC PDUs which are either 136 or 120 bits long. SRBs using acknowledged mode radio link control (AM RLC) produce RLC PDUs which are 128 bits long. In both the unacknowledged and acknowledged modes using the common control channel (CCCH) one or two protocol data units can be transmitted per TTI.
A typical dedicated traffic channel (DTCH) RLC PDU size is 320 bits. As the typical TTI for FACH is 10 ms a single DTCH RLC PDU (or packet) transmitted per TTI uses up all of a 32 kbps data rate capacity of the FACH alone.
The reliability of the forward access channel (FACH) is also limited since retransmissions take a considerable amount of time as retransmissions are carried out on the RLC based on the RLC status indicators transmitted on the random access channel in the uplink. In addition a message transmitted on CCCH does not have any retransmission on the RLC layer and in the case of signalling error the RRC layer needs to initiate retransmission of the RRC message if the appropriate response message is not received within a certain time. This time is typically very long (in the order of seconds), due to transmission delays in the FACH (DL) and RACH (UL) channels.
The typical 3G UE power consumption in the dedicated channel state (CELL_DCH) is approximately 250 mA, in the transitional forward access channel state (FACH) is approximately 120 mA, and in the paging channel state (CELL/URA_PCH) or in the idle state is typically <5 mA. The use of the FACH channel to transmit data can result in a higher power consumption as the forward access channel (FACH) reception requires more time to receive all of the (slow speed) data.
Therefore, in summary, the requirement to use the forward access channel (FACH) over the secondary common control physical channel (S-CCPCH) for transmission (either as an transitional state or as the operating state for passing data) are those of low data rates, slow retransmission rates, and also a relatively high UE power consumption.